1. Field of the Invention
The present invention relates to improvements to power supply backup circuits used for backing up the operation of a circuit, for example, of a radio device when a power supply is interrupted.
2. Description of the Related Art
FIG. 1 is a circuit diagram of a conventional power supply backup circuit. As shown, the circuit includes a reverse current preventing diode 4 inserted between a DC source 1 and a voltage regulator or stabilizer 2 and a capacitor 3 having a capacity large enough to backup a load 5 on the regulator 2, the capacitor having one end connected to a line to which the diode 4 and the regulator 2 are connected and the other end grounded. A load 6 is a circuit, that does not require backup when the power supply is interrupted temporarily.
In operation, when the power supply is stopped, for example, by an instantaneous interruption of the DC source 1, electric charges stored in the capacitor 3 discharge to backup the operation of the load 5 for a predetermined time. At this time, the current of electric charges to the load 6 is prevented by the diode 4.
The capacitor 3 is, a small-type large-capacity tantalum electrolytic capacitor which has excellent leakage current, frequency and temperature characteristics and which has recently been used in response to a demand for miniaturized high-density integrated electronic devices.
However, the tantalum electrolytic capacitor is likely to deteriorate due to rapid charging and discharging, when it is subjected to an excessive voltage or a reverse voltage. It also tends to be destroyed by shorting. Since there is a set load during discharging, discharging creates no problem. The breakage of the capacitor is due mainly to excessive current flowing therethrough during charging.
One possible solution to this problem, is to provide a resistor between the capacitor 3 and the junction point between the diode 4 and regulator 2 to limit the current charging the capacitor 3. However this causes a new problem because during discharging, a voltage available from the load 5 is reduced due to a voltage drop across the resistor in such an arrangement.
As described above, the capacitor 3 is charged and discharged using the single current path 11 in the conventional power supply backup circuit, so that a voltage available from the load 5 is reduced by inserting a resistor in the current path 11 to limit the current charging the capacitor 3.
Thus, use of a tantalum electrolytic capacitor suitable for high-density integration as the capacitor 3 does not provide enough voltage to the load.